1. Field of the Invention
The present invention relates to a method of evaluating the depth of penetration of light into a measurement target as using an optical tomographic image, a performance test method using the evaluation method, and an optical tomography apparatus.
2. Description of the Related Art
In the development of cosmetics and medicines and in medical care, it is important to observe the condition inside the skin. In Europe, since the sale of cosmetics tested on animals has been completely prohibited in the spring of 2013, there is an increasing need for a method for observing the human skin in a non-destructive and non-invasive manner. It is also important as an industrial management method to measure a multilayered structure, such as a photographic film, in a non-destructive manner with high resolution.
Magnetic resonance imaging (MRI), ultrasound, X-ray Computed Tomography (CT), and Optical Coherence Tomography (OCT) are examples of a non-destructive and non-invasive tomographic imaging method. The measurement depth of MRI, ultrasound, and X-ray CT is mm to cm order, which is deep, but the resolution is 10 μm to 100 μm or more, which is not high. OCT is a tomographic imaging method using optical interference. In general, near-infrared light (wavelength of 1.3 μm or 1.5 μm) is mainly applied to fundus examination and the like.
Since the depth resolution Δz of OCT is given by Δz=2×(ln(2)π)× (λ2/Δλ), it is possible to increase the resolution by increasing the wavelength width Δλ of a light source to be used and decreasing the center wavelength λ of the light source (where λ is the center wavelength of the light source, Δλ is the wavelength width of the light source, and Δz is a resolution in the depth direction). In the near-infrared OCT, the depth resolution is about 20 μm. In recent years, in order to improve the depth resolution, development of OCT using the entire wavelength region from visible to near infrared has been performed (Journal of Biomedical Optics, Vol. 9, (1), pp. 47-74, 2004). In Journal of Biomedical Optics, Vol. 9, (1), pp. 47-74, 2004, the depth resolution has reached submicron order.
JP2013-108766A has proposed a method in which low coherent light of red, green, and blue in the visible region is generated by Super Luminescent Diode (SLD) light sources of respective colors and a foundation is applied to skin replica to evaluate the surface unevenness or the thickness of the foundation layer.
JP2014-506510A has proposed using light in a wavelength range of 300 nm to 500 nm as sample light in order to increase the resolution in the depth direction in a known OCT apparatus for ophthalmology that uses infrared light (wavelength of 800 nm to 1300 nm) as sample light.
In JP2008-304314A, a xenon lamp, a light emitting diode, a super luminescent diode, or a multi-mode laser diode that can respond to a continuous spectrum from the ultraviolet region to the infrared region (wavelength of 185 nm to 2000 nm) is exemplified as a light source.
On the other hand, it is known that, in the case of ultraviolet light (wavelength of 400 nm or less), an effect of eliminating the tension of the skin by making the melanin pigment present in the epidermis of human skin proliferate and damaging the collagen present in the dermis occurs. The depth of penetration of ultraviolet light (hereinafter, also referred to as Ultra Violet (UV) light) into the skin is said to be 50 μm to several hundred μm. In the past study example (Photochemistry and photobiology, Vol. 40, (4), pp. 485-494, 1984), the epidermis of actual human skin is cut out and the transmittance of ultraviolet light is measured while thinly cutting the epidermis while changing the thickness, thereby determining the attenuation rate of ultraviolet light at each thickness.